Question

Given, $20ml$ of $0.5N$ HCl and $35ml$ of $0.1N$ NaOH are mixed. The resulting solution will:
A.Be neutral
B.Be basic
C.Turn phenolphthalein solution pink
D.Turn methyl orange red.

Answer 1

At first we will look at the given substances and then we will see what a normality is. Then we will calculate the milli equivalents for both HCl and NaOH. Then we will understand the nature of reaction of happening here and how many milli equivalents are consumed of which substance. The substance which will not be entirely consumed will give us over result.

Complete answer:
Step1. We have given HCl. It is a strong acid.
Volume of HCl: $20ml$
Normality of HCl: $0.5N$
Another compound is NaOH
Volume of NaOH: $35ml$
Normality of NaOH: $0.1N$
Step2. Normality is used to measure the concentration of a solution. It is represented by $N$. It is equivalent to a concentration of solution.
$Normality = \dfrac{{eq.grams}}{{volume}}$
Step3. We calculate the equivalent of HCL
$20ml$of $0.5N$ HCl = $20 \times 0.5 = 10millieq$
We will calculate equivalent of NaOH
$35ml$ of $0.1N$ NaOH = $35 \times 0.1 = 3.5millieq$
Step4. The equation here is the acid base reaction. It will produce a salt and water
$HCl + NaOH \to NaCl + {H_2}O$
From here we can say that one HCl consumes one equivalent of NaOH.
Step5. $3.5millieq$ of HCl will consume the same $3.5millieq$ of NaOH.
So the $6.5millieq$ of HCl will be left.
Hence the solution left is acidic.
Step6. Now an acidic solution always turns the methyl orange red. It gives a positive test to it.

So option D is the correct answer.

Note:
There are different acid-base indicators. As acid converts Methyl orange into red the base changes it into yellow, There is also litmus test for acid base indicator. Blue litmus turns to red under acidic solution and the red litmus turns blue under basic solution.